1. Field of the Invention
The present invention relates to an apparatus for calculating coordinate data of a desired point in a subject to be examined which is required when stereotaxy (stereotactic surgery) or the like is to be performed by using a computed tomographic apparatus such as an MRI apparatus (Magnetic Resonance Imaging Apparatus), and a method of calculating coordinate data of a desired point in a subject to be examined by using the same.
2. Description of the Related Art
Coordinate data of a desired point in a subject to be examined is used, for example, for stereotaxy surgery as one of operations of neurosurgeon. In stereotaxy, the position of a morbid portion in the brain of a patient is identified with the aid of an image, and an operation is performed, e.g., inserting a biopsy cannula in the identified portion so as to remove the morbid portion. As the image, an X-ray image (Roentgen image) obtained by an X-ray diagnosing apparatus has been used. Recently, however, CT stereotaxy using an X-ray CT apparatus as a computed tomographic apparatus and MRI stereotaxy using an MRI apparatus as a computed tomographic apparatus have been performed.
MRI stereotaxy is conventionally realized by the coordinate calculation step of calculating the coordinate value of a morbid portion in a subject to be examined, and the step of operating a stereotactic apparatus on the basis of the calculated coordinate value, i.e., the operation step. FIG. 1 shows an apparatus used in the coordinate calculation step and a subject to be examined. FIG. 2 shows an apparatus used in the operation step, i.e., a stereotactic apparatus and the subject.
As shown in FIG. 1, in the apparatus used in the coordinate calculation step, a head portion 40A of a subject 40 to be examined is held in an annular frame 20. In this case, an indicator phantom 21 is mounted on the frame 20 in order to indicate X-, Y-, and Z-coordinates. The frame 20 is placed at a predetermined position in a photographable region of a computed tomographic apparatus (not shown) while the head portion 40A is held by the frame 20, and a photographing operation is performed to obtain, e.g., an axial plane image. By observing this image, the position of the morbid portion appearing on the image is recognized and identified with X-, Y-, and Z-coordinates by referring to the indicator phantom 21. The indicator phantom 21 is detached from the frame 20, and a stereotactic apparatus 22 is mounted. Thereafter, for example, a biopsy cannula 22A of the apparatus 22 is pierced into the head portion 40A to reach the position coordinates of the morbid portion obtained in the above described step as a target, and the morbid portion is removed.
In the execution of the above-described stereotaxy, since a surgical operation is to be performed for a brain, high positional precision is required in identification of a morbid portion.
In an MRI apparatus, however, because of the characteristics of static and gradient fields, field distortion is abruptly increased with an increase in distance from the field center, and following this field distortion, image distortion is abruptly increased. Example, image distortion corresponding to 5 mm may be caused at a position near the frame, thus posing a problem in execution of stereotaxy requiring high positional precision.